The present invention is generally related to a glare protection device for welding protective masks and to a method for controlling a glare protection device. The present invention is further directed toward a glare protection device including an active electro-optical protective filter, an electronic circuit, and an optical sensor, and toward a method for controlling such a glare protection device.
Glare protection devices of the aforementioned type are generally known from, for example, WO98/57606 or the EP 0 550 384, wherein an optical sensor for the detection of the welding light by means of an active electronic circuit switches an active electro-optical protective filter such that, when welding light occurs, immediately a predefined blacking out level (protection level) is switched on. Similarly, when the welding light stops, the blacking out is immediately switched off again.
Modern glare protection devices of this kind, in particular as sight window for welding protective masks, as the active filter element typically include a liquid crystal cell, which blocks the passage of light to a greater or lesser extent, as soon as the light intensity detected by a sensor exceeds a predefined threshold. In doing so, an electronic circuit in the glare protection device comprises an evaluation circuit for the sensor output signal and a driving circuit for the liquid crystal cell.
For the detection of the radiation up until now usually silicon photo detectors have been utilised. Silicon photo detectors detect radiation in a wavelength range from red to the near infrared (NIR), i.e., within the range of, e.g., 600-900 nm. Every welding process has its own characteristic light spectrum, which, for example, is determined by the welding parameters (electric current, gas, materials) and the welding process. In the case of practically all welding processes, the proportion of the emitted radiation in the UV range is great, while in the red—and NIR range a relatively small proportion is emitted. On the other hand, however, by extraneous light, e.g., by artificial light, a relatively large proportion is emitted in the long-wave range, i.e., in the NIR—and red range, while the UV proportion of artificial light is relatively small. For this reason, within the sensitivity range of the silicon detectors utilised up to now a relatively small proportion of the welding light and in contrast, however, a great proportion of extraneous—and stray light is detected, which renders the differentiation of the welding light from this stray light exceedingly difficult.
In order now to be in a position to differentiate the welding light from the extraneous light and from any possible stray light effects for the purpose of driving the protective filter, a very elaborate electronic circuit is required. In this, in particular to so-called flickering light from the electric welding arc is utilised for the separation of the welding light from extraneous light. In the case of an insufficient differentiation of the welding light from the extraneous light a malfunction occurs, i.e., that the blacking out does not switch on when welding light occurs, because the extraneous light sources are dominant. Similarly, insufficient differentiation also may prevent the blacking out from being switched off when the welding light ceases. In the case of the evaluation circuits in use today therefore so-called flickering light circuits are made use of, which detect the flickering light of the welding light within a certain frequency range, filter it out, and utilise it for the evaluation. The signal to noise ration here, however, is very poor, so that flickering light circuits of this type are exceedingly sensitive, elaborate, and subject to interferences.